This invention relates to a telephone communications interface coupler and, in particular, an optically coupled 2 wire to 4 wire coupler comprising two optical coupled devices which are capable of coupling signals to and from a telephone line while maintaining circuit isolation and automatic gain control.
Telephone systems worldwide include a DC current which is overlaid with an AC signal current. This DC current is generally provided utilizing 48 volt batteries which are located at the telephone central office. Depending upon the distance of the particular user from the central office, the current from that 48 volt battery can range, as for instance, from about 5 ma to about 120 ma.
In PBX systems and other switching systems, it is necessary to utilize outside power supplies to drive these pieces of equipment. Because the DC current, which is provided by the central office of the telephone company, is grounded differently than the outside power supplied to the PBX or other switching systems, it is necessary to isolate these two circuits in order to avoid erroneous operations of the telephone system. Inductive telephone 2 to 4 wire transformers are currently the only known devices which are suitable for use in isolation of these two electrical circuits.
In certain instances, optical coupled devices have been utilized for circuit isolation. However, because of the variability of the telephone DC currents and the loading on telephone networks, the various standard techniques incorporating optical couplers for audioo transmission and/or isolation have not proven to be useful in telephone networking. Wile transformers have been successfully utilized in telephone network, they have certain inherent defects because of their physical structure and size. By their very nature, they are susceptible to outside magnetically induced spurious noises and they are bulky and heavy. In certain communication interfaces, as for instance in modems and the like, the physical bulkiness of the transformers is considered disadvantageous.
As telephone communication systems become more complex, it has become desirable and necessary to reduce the physical size of the same and reduce the number of components so as to reduce the cost and complex of the device, yet maintaining and improving the performance.